Lubricant fuel/energy efficiency will be an important feature for future automotive engine lubricants and commercial vehicle engine lubricants. For automotive engine lubricant formulations, it is generally preferred to have lower viscosity fluids, e.g., below about 15 cSt at 100° C. Lower viscosity is known to impart lower viscous drag thus offering better energy efficiency or fuel economy. It is also important to have a lubricant formulation with a low friction coefficient. Fluids with low friction coefficients exhibit low frictional loss during lubrication. Low frictional loss is important for improved fuel or energy efficiency of formulated lubricants. Moreover, improved antiwear properties and film thickness are beneficial for increasing the lifetime of engine components. Another important property for engine lubricants is low pour point.
Polyalkylene glycol (PAG) fluids have been employed as lubricant base stocks. PAG fluids possess performance advantages that provide good efficiency, including very low friction/traction for energy efficiency and good lubricity (in hydrodynamic, mix, and boundary lubrication conditions). PAG fluids also have other desirable properties, including high viscosity index (VI), low pour point, and excellent cleanliness. PAG fluids, however, have numerous drawbacks, including lack of miscibility and compatibility with mineral and synthetic hydrocarbon-based lubricants. This has limited their use in conjunction with such base stocks. PAG fluids are also polar and highly soluble in water, which can result in severe corrosion problems. Moreover, the formulation or additive response of PAG fluids can be unpredictable, rendering them difficult to formulate with.
Polyethylene glycol, polypropylene glycol and polybutylene glycol, for example, are PAG fluids that are soluble in water, but very slightly soluble (e.g., less than about 0.1 wt % at 23° C.) in mineral and synthetic hydrocarbon-based base stocks. Each of these PAG fluids contains two OH groups which may contribute to their reduced solubility in non-polar solvents such as hydrocarbon-based base stocks.
As an example of a PAG, the structure of polyethylene glycol is shown by the following formula:

L. Rudnick and R. Shubkin, in Synthetic Lubricants and High-Performance Functional Fluids (2d Ed. 1999), Chapter 6, Polyalkylene Glycols, pp. 159-193 (“Rudnick”), describe polyalkylene glycols and their use as lubricants. Rudnick describes types of polyalkylene glycols used commercially as lubricants including, among others, “[h]omopolymers of propylene oxide (polypropylene glycols), which are the water-insoluble type” and “show limited solubility in oil”, such as “monobutyl ethers” (p. 163). Rudnick also describes “[c]opolymers of ethylene oxide and propylene oxide, which are the water-soluble type” and “are typically diols or monobutyl ethers” (p. 163). Rudnick also describes “[p]olymers of butylene oxide [which] show greater oil solubility than the homopolymers of propylene oxide,” “[p]olymers of propylene oxide and higher epoxides designed to give greater oil solubility” and “[p]olymers of propylene oxide that are dimethyl ethers” (p. 164). Other references which discuss polyalkylene glycols and related compounds are: U.S. Pat. Nos. 4,973,414, 5,024,678, 5,599,100, 5,746,933, 6,087,307, U.S. 2003/0104951, U.S. 2009/0107035, U.S. 2010/0004151, U.S. 2010/0093572, EP 355 977, EP 524 783, EP 246 612A, WO 2000/23544, JP 54159411A, JP 61166892, JP 6179888, JP 6128580.
It would be desirable to have mineral and synthetic hydrocarbon-based lubricant compositions that take advantage of the desirable qualities of PAG fluids, including their good frictional properties, high VI and cleanliness, while overcoming their drawbacks, including the lack of miscibility of PAG fluids with mineral and synthetic hydrocarbon-based lubricants and unpredictable additive response. It would also be desirable to improve the pour point of such lubricant compositions.